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Joule Heating of Metals with Different Resistivities

  1. Sep 3, 2014 #1
    Hello,

    I am a high school student carrying out a physics experiment.

    I created a short circuit in order to see how much heat different types of metals would release when a current is running through them (4.5 A). In my experiment, I tested three different types of metals: Brass, Stainless Steel, and Aluminium.

    Stainless Steel 304 has a resistivity of2.84∗ 10^(-7) Ohm meters(AK Steel.).

    Aluminium 6063 has a resistivity of 3.32 ∗ 10^(-8) Ohm meters("ALCOA 6063.").

    Free Machining Brass 3604 has a resistivity of 5.9 ∗ 10^(-8)Ohm meters(The Engineering Toolbox. "Resistivity, Conductivity and Temperature Coefficients for Some Common Materials." )

    I immersed 4 cm of these three different metals all with the same diameter of 3mm into 20ml of vegetable oil and observed the change in temperature by using a thermometer. The container with the vegetable oil where the experiment was conducted was surrounded by a layer of glass fiber to insulate it.

    Due to Joule's First Law, which is that Q=RtI^2
    (R= Resistance, t=time period, I=current)
    In my experiment, I kept current and time period constant.

    I expected that:
    Heat Released by Stainless Steel> Heat Released by Brass> Heat Released by Aluminium

    However, when I actually did the experiment,
    Heat Released by Stainless Steel>Heat Released by Aluminium>Heat Released by Brass

    Can anyone offer an explanation for this?

    Thank you!
     
  2. jcsd
  3. Sep 3, 2014 #2

    BvU

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    Gold Member

    Hello Steve, and welcome to PF!

    So qualitatively things didn't go as you expected. It happens; welcome to physics in general...

    Maybe you did more than three measurements and varied a few things ? Everything reproduces exactly ?

    You should also be able to do some quantitative work, which you then might want to share with us.


    Perhaps then we can estimate if this is just bad luck (within the experimental errors) or really a surprising result !
     
  4. Sep 3, 2014 #3

    phyzguy

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    Science Advisor

    Did you try calculating the total resistance of these pieces of metal? If you do, you will find that the resistance is very small, probably less than .001 Ohm. Given this, the resistance of the wires connecting to the pieces of the metal, and the resistance of the contacts between the wires and the pieces of metal you are testing is probably greater than the resistance of the metal under test. What if you try measuring the total resistance of the circuit to see if this is the case?
     
  5. Sep 7, 2014 #4
    BvU: Hi, I repeated the experiment two times, but the results were the same.

    The Aluminium Wire caused the vegetable oil to heat from
    24.7 degrees to around 28 degrees

    Stainless Steel
    24.7 degrees to around 31

    Brass
    24.7 degrees to around 27

    I tried to find other materials, which would come in the same form, but its really hard, as they are solid wires.

    phyzguy: I did not try to calculate the total resistance of the metals, but I realise that its resistance would be very small. I will try to find out the total resistance of the circuit, but as I saw a clear temperature difference in the different metals, I am not sure what this would achieve...

    I am sorry for the late reply and thank you for commenting!
     
    Last edited: Sep 7, 2014
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